Here is a really terrible mockup of what I envision the New Image dialog looking like eventually:

http://pinta-project.com/temp/new-dialog-mockup.png

It provides the following features:
- New image from clipboard size
- Some common presets
- What the initial background should be (transparent/white)

Jonathan, you could also add "Don't show this dialog anymore" checkbox
https://bugs.launchpad.net/pinta/+bug/762860

From "Clipboard" is nice idea, but maybe it should also have "From Clipboard and paste". The main idea is if someone likes to have a new image like size from clipboard then he/she likes to paste it from clipboard, isn't it.

Maybe you should add another option like "Screen size" - you know dependent of the screen size of user - if user have a large screen, he/she will probably like to have a large image (maximum what he/she can see on the screen), but if you have a small screen you would need small canvas.

I am having this problem, because I am using Pinta on notebook that is having 1024x768 and on very big LCD screen on PC. I like to have a canvas as big as possible.

Maybe you could also add "history" section. For example I select 700x500, then I select 600x400, etc. Now I have to manually enter this value every time.

Maybe adding measure feature. Now every settings is in pixels, but a lot of new users probably don't know what pixel is. So having inches, centimeters etc can help much. Paint.net has this feature.

I wrote the code fix for this, but don't know how to submit it, and GDK isn't letting me compile on OS X at the moment.

But this is the code to insert into Actions/File/NewDocumentAction.cs

        Gtk.Clipboard cb = Gtk.Clipboard.Get(Gdk.Atom.Intern ("CLIPBOARD", false));
        if (cb.WaitIsImageAvailable()) {
            Gdk.Pixbuf image = cb.WaitForImage();
            dialog.NewImageWidth = image.Width;
            dialog.NewImageHeight = image.Height;
        }
        else
        {
            dialog.NewImageWidth = PintaCore.Settings.GetSetting<int> ("new-image-width", 800);
            dialog.NewImageHeight = PintaCore.Settings.GetSetting<int> ("new-image-height", 600);
        }

fixed on github pinta project